This invention relates to the calibration of gamma sensors and more particularly to in-reactor recalibration of such instruments.
Much effort has been devoted to in-reactor calibration of power sensing instruments not capable of being calibrated outside the reactor. Because such instruments require frequent in-core recalibration, expensive traveling fission chamber systems involving traveling in-core probes are utilized to relate signals from many fixed sensors to the signal from a single sensor positioned adjacent to each and every fixed sensor in the reactor. While many local nuclear power measuring devices must likewise be frequently recalibrated, the power measuring devices disclosed in prior copending application, Ser. No. 888,881, filed Mar. 21, 1978, now U.S. Pat. No. 4,298,430 issued Nov. 3, 1981 are so constructed as to provide for calibration at the point of manufacture. In this type of power measuring device, radial heat flow paths extend through a gamma radiation heated body between the internal surface of an inner bore, housing the thermocouple sensors, to the external cylindrical surface of the body. Thermal resistance gaps in the external surface of the body produce localized directional changes in the radial heat flow paths as explained in the aforesaid patent to thereby produce differential output signals from the thermocouple sensors. Thus, such power measuring devices feature an elongated heat conductive body that is also electrically conductive to enable electrical resistance heating to be utilized for calibration prior to installation in a nuclear reactor. Nevertheless, recalibration is still desirable for such pre-calibrated instruments to ensure that sensitivity remains constant. It would therefore be desirable to remove such power measuring units from the reactor periodically to repeat the same calibration procedure performed prior to installation. However, such recalibration procedure is not practicable because the unit becomes extremely radioactive after in-reactor service and cannot be safely handled.
It is therefore an important objective of the present invention to provide economical apparatus and method for recalibration of power measuring units that have been precalibrated by resistance heating without removal from a nuclear reactor installation.